The present invention relates to a data recording and reproducing apparatus, or more in particular, to a data recording and reproducing apparatus in which data is capable of being recorded or reproduced on the one hand and is capable of being erased on the other hand with a laser beam radiated on an optical disc.
FIG. 5 shows a phase change between an amorphous state A and a crystal state C of a phase-changed recording medium of a conventional data recording and reproducing apparatus. The reflection factor is small in the amorphous state A, and high in crystal state C. The crystal state C is reached when the temperature of a medium is increased locally to almost the melting point and the particular part is annealed, whereas the crystal state A is attained when the temperature of the part in the crystal state C is locally increased to almost the melting point followed by quenching. FIG. 6 is a diagram showing the principle of recording and reproduction for explaining the signal recording and reproduction operation. FIG. 6 (a) shows a construction of a light beam for realizing the condition for temperature increase followed by quenching and temperature increase followed by annealing of the medium. FIG. 6 (b) shows a light distribution for that purpose. In FIG. 6, reference numeral 9 designates a recording-reproducing light beam with small diameter, numeral 12 a distribution thereof, numeral 10 an erasing light beam with long diameter, and numeral 13 a light distribution thereof. Numeral 11 designates a guide track formed by evaporation with the recording medium. The difference in light beam diameter is such that the conditions for temperature increase followed by quenching are set up with a short diameter, and the conditions for temperature increase followed by annealing are established with a long diameter. FIG. 6 (c) shows waveforms of reproduction signals from an identifier section (hereinafter referred to as "the ID section"), a data section and a gap section of a sector formed in the guide track 11 of the optical disc.
FIG. 6 (d) shows a write gate signal 105 for setting the recording and reproducing light beam in a recording mode and commanding the writing by modulating it with a write data into the data section, and FIG. 6 (e) an erasing gate signal 106 for controlling the period of time during which the erasing light beam 10 is radiated on the data section of the guide track 11 with a predetermined intensity. In the data section, the write gate signal 105 and the erasing gate signal are enabled so that the data section is erased with the preceding erasing light beam 10, and the write data is recorded by the succeeding recording-reproducing light beam 9. At the end of the data recording, the write gate signal 105 and the erasing gate signal 106 are both turned off.
In the above-mentioned configuration, the write gate signal 105 and the erasing gate signal 106 are turned off at the same time, and therefore, the operation of writing in the medium is effected with the erasing light beam 10 in the manner shown by 14 of FIG. 6 (c).
The medium is erased by increasing the temperature and annealing by the erasing light beam 10. This process of temperature increase followed by annealing is realized as a whole function including the peak at the leading portion and the uniform distribution 13 at the rear portion of the erasing light beam 10. The very instant the erasing gate signal 106 is turned off, therefore, at the leading portion of the distribution 13 of the erasing light beam 10, the portion of the medium beyond the recording threshold light power Pt meets the conditions for temperature increase and quenching thereby to render the recording as shown by a recording trace 14 in view of the fact that the light at the rear portion of the erasing light beam 10 in the distribution 13 is turned off and is not radiated.
This recording trace 14, which is a jitter caused by the eccentricity of the disc or the rotation variations thereof, varies from several tens of microns to 100 microns in position. Further, since the preceding recording traces are erased only partially, it makes up a signal having much DC components 10 microns wide after a multiplicity of erasures. As a result, the reproduction signal has a waveform distortion such as zag caused by DC component change in the gap, thus posing the problem that the ID section of the next sector cannot be read with high accuracy. Also, the erasing light beam 10, which is radiated all the time, has a higher average power than the recording power modulated, and therefore if it is turned off suddenly as explained above, a thermal shock is exerted on the medium. As a consequence, at the turned-off portion of the erasing light beam 10, the effect of the thermal expansion caused in the recording medium of the data section of the sector causes a separation from the base material of the recording film, or the repetitive recording and erasure causes a conspicuous thermal fatigue thereby to deteriorate the recording-reproduction characteristics.